The present invention relates to a sensor circuit. More particularly it relates to such a sensor circuit which has a measuring circuit producing a signal depending on a variable value and an evaluating circuit, which leads to the fact that the signal given by the measuring circuit is for example to be smoothed or to be amplified.
Measuring circuits and especially so-called differential measuring circuits are important within the sensor circuits. They are formed as measuring circuits in which measuring voltages occur at two different switching points, and the signals at one point change in opposite direction to the signals at the other point during the change of the measuring values. The conventional measuring circuits of this type are all modifications of bridge circuits. Further examples of these measuring circuits are so-called differential trafo displacement pickups and differential throttle displacement pickups. The decisive trick in the known differential measuring circuits is that the difference between the above mentioned two measuring voltages is formed. Thereby a signal is produced whose change during a predetermined change of the measuring value is double the size of the change for one or both measuring voltages. Thereby this signal/noise ratio is improved. Moreover, non-linearities and temperature fluctuations of the individual measuring voltages are eliminated by the differential formation.
Due to the above mentioned advantages the conventional differential measuring circuits are normally used where reliable and accurate measurement is required. In such cases it is frequently important that the operability of the sensor circuit in which the differential measuring circuit is arranged is to be monitored. This is performed by doubled execution of the sensor circuit and by comparison of the sensor signals produced by both sensors. When the signals are not plausible to one another an error signal is produced.
It is generally desirable in practice to design such sensor circuits so that they are as simple as possible and therefore form reliable devices.